A prior art accelerometer with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer comprises three primary components, a proof mass assembly, and upper and lower stators or magnetic circuits between which the proof mass assembly is secured. The proof mass assembly includes a movable paddle that is suspended by flexures from a support ring that extends around the paddle, such that the paddle can pivot with respect to the support ring about a hinge axis that passes through the flexures. The paddle, flexures and support ring may be fabricated as a unitary structure composed of fused quartz. A plurality of mounting pads are formed on the upper and lower surfaces of the support ring. These mounting pads contact inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
In the accelerometer of U.S. Pat. No. 3,702,073, each stator includes a permanent magnet, and each surface of the paddle has a force balancing coil mounted on it. Current flowing through each coil produces a magnetic field that interacts with the permanent magnet of the associated stator, to produce a force on the paddle. By controlling the electrical currents supplied to the coils, one can control the magnitude and direction of this force. The upper and lower paddle surfaces also include capacitor plates configured to form capacitors with capacitor plates on the inwardly facing surfaces of the stators. Movement of the paddle with respect to the upper and lower stators results in a differential capacitance change that can be sensed to determine the paddle position.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along a sensing axis normal to the paddle results in rotation of the paddle about the hinge axis with respect to the support ring and stators. The resulting differential capacitance change caused by this movement is sensed by a feedback circuit. In response, the feedback circuit produces a current that, when applied to the force balancing coils, produces a force that tends to return the paddle to its neutral position. The magnitude of the current required to maintain the paddle in its neutral position provides a measure of the acceleration along the sensing axis.
In a second type of prior art accelerometer, the coils and capacitor plates are replaced by one or more force sensing elements connected between the paddle or proof mass and a fixed support. When the object to which the accelerometer is affixed accelerates along the sensing axis, the paddle movement produces a force that is detected by the force sensing elements. The output signals of the force sensing elements may then be processed to provide a measure of the acceleration.
In accelerometers of the types described above, in which a support ring is secured between upper and lower stators or other mounting members, it is often desirable to clamp the support ring only at that portion of the ring that is spaced away from the flexures, so that the portion of the support ring adjacent the flexures is not directly clamped. The advantage of such a support technique is that stresses exerted on the proof mass assembly by the stators are not directly transmitted to the flexures, where they could cause bias errors.
When accelerometers are used in environments in which they may be subjected to high acceleration impacts, such as in oil field survey equipment, provisions must generally be made to prevent such impacts from fracturing the flexures, or from otherwise damaging the accelerometer. For example, along the sensitive axis of the accelerometer, shock stops are commonly provided to prevent excessive motion of the paddle along the sensitive axis. In cross axis directions, i.e., in directions normal to the sensing axis, it has been known to provide a pin that is attached to a stator or other support, and that passes through an opening in the paddle. The opening is sized such that the paddle is allowed limited travel in cross axis directions, and in particular along the hinge axis of the accelerometer, to thereby protect the flexures from breakage. However, despite such measures, it has been found that such accelerometers remain subject to failure after extended use in the field. Two particular types of failures that have been noted are breakage of the flexures, and of the unclamped portion of the support ring in the general area of the flexures. An additional failure mode involves breakage of the thin wires that extend from pins fixed within the accelerometer onto the unsupported support rim portion, for conducting signals to and from the coils and capacitor plates.